1. Field of the Invention
The present invention relates to a variable-capacitance circuit element and device, which may be used in particular in an oscillator with frequency controlled by a voltage.
2. Description of the Related Art
Certain applications of electronic circuits require a variable-capacitance capacitor. It is known to produce such a capacitor in the form of a circuit element consisting of two varactors connected in series. FIGS. 1a and 1b represent the production of such a variable-capacitance circuit element. The two varactors are produced in MOS technology (standing for “Metal-Oxide-Semiconductor”). An N− doping well, referenced 202, is formed in a P type semiconductor substrate, referenced 200. The well 202 is surrounded by an electric insulation barrier 201, for example of the STI type (standing for “Shallow Trench Isolator”). A central zone 203 and two lateral zones 204a and 204b, situated in the well 202, have a strengthened doping of N type, denoted N+. A first MOS gate, referenced 205a, is formed on the surface S of the substrate 200 between the zones 203 and 204a, and a second MOS gate, referenced 205b, is formed on the surface S between the zones 203 and 204b. The gates 205a and 205b are electrically insulated from the well 202 by insulating layers 206a and 206b, respectively. Each of the two varactors, referenced 10a and 10b, therefore corresponds to a modified structure of MOS transistor, in which the channel zone on the one hand, and the source and drain zones on the other hand, have dopings of the same type but of different concentrations of doping species. The two varactors 10a and 10b are connected in series by the common zone 203. Preferably, they are identical. Together they constitute the variable-capacitance circuit element 1.
The zones 203, 204a and 204b are connected electrically by respective connections 13, 14a and 14b which are linked together. These connections form an intermediate link for arranging the varactors 10a and 10b in series (see FIG. 1b). The two gates 205a and 205b are connected electrically by respective connections 12a and 12b and form the two outputs of the circuit element 1.
A bias terminal 11 links moreover an additional N+ doped zone, situated in the well 202. This terminal constitutes the input of the circuit element 1. The capacitance of the circuit element 1, measured between the outputs 12a and 12b, varies as a function of an electrical control voltage C1 applied to the input 11.
Such a circuit element makes it possible to obtain minimal variations in capacitance which are of the order of 30 aF (aF standing for attoFarad, 1 aF=10−18 F), for present-day MOS technologies. Now, these minimal variations are too big for certain applications. In particular, an oscillator with frequency controlled by a voltage, or VCO standing for “Voltage Controlled Oscillator”, which is suitable for use with an inductance of 1 nH (nanoHenry) approximately in a radio transmitter, requires a frequency spacing of the order of 100 Hz (hertz) to 1 kHz (kilohertz). Such a frequency spacing corresponds roughly to a capacitance spacing of the order of 1 to 10 aF.